26 



A TYPICAL VERTEBRATE EYE: THE HUMAN 



lens. If we move the screen toward or away from the lens the image will 

 immediately become blurred because the object-points will be represented 

 on the screen not by sharp image-points, but by patches of light of the 

 same shape as the lens ('blur' or 'confusion' circles, where the lens is 

 round) which overlap each other. 



If the screen now remains stationary at the proper distance, and the 

 object moves toward or away from the lens, the image will focus behind 

 or in front of the screen (Fig. 11), and the picture on the latter will 

 again be composed of hazy blur circles. With the object in this new 

 position, its image can now be made to fall on the screen only if the lens 

 is shifted in position or altered in curvature. Both of these methods are 

 used, in different kinds of vertebrate eyes, to keep the image sharp on 



Fig. 11 — Relation of objea-distance to image-distance. After Kahn. 



Only the B is sharply imaged on the screen, on which the A and C are represented by blurs. 

 The sharp images of the A and C hang in space as shown, and can be placed on the station- 

 ary screen only by moving the lens, or by substituting another lens of different strength. 



the retinal screen when the object varies in distance from the eye. These 

 adjustments comprise what is called 'accommodation'. 



Refractive Errors of the Eye — In the human eye there are several 

 curved surfaces at which refraction takes place, the end result being the 

 production of an image on the retina. There is also an elaborate arrange- 

 ment for changing the curvature of one of these surfaces so that the 

 image can be moved slightly forward or backward in the eye. This 

 mechanism of accommodation comes into play when we shift our gaze 

 from a distant to a nearby object, or when we watch an object which is 

 moving toward or away from us. As an object approaches, its image 

 recedes behind the retina and must be pulled forward. As an object goes 

 away from us, its image moves forward into the vitreous and must be 

 pressed back onto the retina in order to be seen sharply. In many persons 

 the eyeball is abnormally short (Fig. 12, top diagrams), so that the 



